In 3GPP2 (3rd Generation Partnership Project 2) of the International Standards Organization, a standardization has been performed for a cdma2000 1x system of a mobile communication system feasible for a voice communication and data communication, and a cdma2000 1xEV-DO (1x Evolution—Data Only) system of a mobile radio communication system, the spectrum efficiency of which is improved by specialization exclusively used for the data communication. The cdma2000 1x and 1xEV-DO communication systems were used so that a unicast communication which communicates with the terminals by one-to-one manner is realized by a mobile network. However, a multicast communication has been studied to realize a communication by one-to-multitude manner. As factors of technique for realizing the multicast communication, a so-called broadcasting channel, as a channel to perform a data transmission, has been standardized by C. S0001-D v1.0, C. S0002-D v1.0, C. S0003-D v1.0, C. S0004-D v1.0, C. S0005-D v1.0 of the 3GPP2 standard used for the cdma2000 1x, and by C. S0054-0 v1.0 used for the cdma2000 1xEV-DO. A service using this broadcasting channel is referred to as BCMCS (Broadcast Multicast Service).
A channel for the unicast having been used for the communication between the base station and mobile devices in the past has a feature so that a single mobile device alone enables to receive data transmitted from the base station, however, the broadcasting channel standardized for a multicast communication support has a feature so that all of the radio wave receivable mobile devices enable to receive the data transmitted from the base station. In the conventional communication method using the channel used for the unicast, a handoff process etc. is performed in the case where a power control and application for a modulation system, which becomes optimum between the mobile device and base station as a communication target, encounter with other base stations, it is therefore possible to control an optimum communication speed and packet error rate in response to a radio reception environment of the mobile device. However, the control using such the system cannot be performed in the case of using the broadcasting channel, since the mobile devices to be communication targets become unspecified. Particularly, an interference increases around areas between the base stations. Therefore, there arises a problem that a high data rate and low error rate communication cannot be performed. For a purpose of solving the problem, a technique called a soft combine is introduced into the 3GPP2 standard C. S0054 (non-patent document 1).
The soft combine has been known as a technique for improving a reception environment of a communication using the broadcasting channel in a condition where the reception environment in a poor reception area has an interference of radio waves transmitted from a base station located between coverage areas of one base station and the other base station. In the soft combine, each of the base stations transmits identical data at the same time by using the same radio parameter. This makes a receiver of the mobile device to receive and synthesize radio waves arrived from the respective base stations, so that the radio waves from the other base stations which are the source of interference can be changed to a gain to improve the reception environment. In order to realize the soft combine, it is necessary to transmit completely identical data synchronized with the same time information from the base stations. In the case of taking an example of cdma2000 1xEV-DO radio interface, the radio interface in a downstream direction is constituted by an aggregate of time slots of 1.67 ms. However, it is necessary to transmit the data synchronized with the same time information by every time slot of 1.67 ms so that the data to be transmitted between the base stations is not deviated. The soft combine becomes enabled even if there is a transmission timing error between the base stations in the case where the mobile devices have a function for absorbing a radio wave arrival time difference caused by the timing error and propagation delay between the base stations. However, even in this case, it is necessary to perform the synchronization for the data transmission so that a constant amount of the timing difference does not occur between the base stations.
As a technique for assuring the timing synchronized with the identical time information, an anchor system applicable with an A3 interface has been disclosed as disclosed in the 3GPP2 standard A. S0015-C v1.0 (non-patent document 2). This anchor system is used for realizing a technique equivalent to the soft combine called a soft handoff in the cdma2000 1x radio. However, in the anchor system, a single base station becomes an anchor base station to aggregate a plurality of base stations. The anchor base station therefore manages the transmission timing of all of the base stations to control the transmission. The anchor base station indicates a transmitting time for every frame of 20 ms which is a minimum unit of the radio transmission timing as a transmitting time, when data is transmitted to respective base stations. It is therefore noted that the anchor base station serving as an aggregation device to aggregate the base stations should manage radio interface information in detail, for calculating the radio transmission timing.
Non-patent document 1: 3GPP2 standard C. S0054
Non-patent document 2: 3GPP2 standard A. S0015-C v1.0